So, I was finally forced to use Linux because of software for a class at school.

I installed Ubuntu, on the partition that used to hold Windows XP. Ubuntu's disk partitioning tool didn't recognize my FreeBSD partition - it said "unknown." I'm not sure if it's the fact that it was unknown, or if I didn't select a mount point for the FreeBSD partition (I don't think that could be the problem, since I didn't want to change anything about my FreeBSD partition). But, now when Ubuntu's GRUB bootloader comes up, I can't select FreeBSD.

The FreeBSD partition is still on my machine, I just can't boot to it. Does anyone know how to fix this?

You have to go into the advanced crap towards the end and tell it to installing GRUB into the Ubuntu / partition, in order to avoid that.

If you want to use GRUB, setting up the menu entry is easy, if you want to reinstall FreeBSDs boot loader; you'll have to install GRUB into Ubuntu's /, then boot off your FreeBSD install disks (or use GRUB to get to FreeBSD) and re-install the boot code via fdisk.

Which way are you wanting to go?

Edit: GAG as J65nko suggested, is also a great boot loader; I've used it for years.